This invention relates to a durable induction coil for heat treating metal workpieces. More particularly, this invention relates to an induction coil formed of copper and comprising a refractory metal cladding protecting a workpiece-facing surface to extend coil life with minimal loss of inductive heating efficiency.
A surface of a steel workpiece may be hardened by electromagnetic inductive heating and quenching. An alternating electrical current through an electrically conductive coil creates a rapidly fluctuating magnetic field, which in turn induces current in a workpiece within the field. This induced current is concentrated near the workpiece surface and resistively heats the surface. Because field strength is inversely proportional to distance from the inductor, it is desired to position the workpiece proximate to the coil, while maintaining an insulative spacing therebetween, to maximize heating but avoid arcing.
Conventionally, the induction coil is formed of copper, which has a low electrical resistivity, typically less than two microohm centimeters for commercial grades, to minimize resistive heating within the coil. However, because of the proximate placement, heat radiated from the workpiece heats adjacent coil surfaces to a temperature sufficient to oxidize the copper. Although typically coolant is circulated through the coil, this has not been entirely satisfactory to eliminate corrosion as a main reason for coil failure. Also, arcing that may occur between the coil and the workpiece pits the copper surface. Occasional collisions while positioning the workpiece near the coil causes wear and further shortens coil life.
It is an object of this invention to provide an improved induction coil having an extended useful life, which coil comprises a protective thin refractory metal lining on workpiece-adjacent surfaces to reduce corrosion, pitting and wear that would otherwise shorten the life of the coil. The lining is formed of a refractory metal having a resistivity which, although not as low as that of copper, is sufficient to minimize electrical losses due to the lining so as to maintain inductive heating efficiency.